The development of the alveolar portion of the mammalian lung take place principally during postnatal life. The alveoli proliferate rapidly during childhood (normal postnatal growth), and there is a rapid burst of growth in the remaining lung following pneumonectomy (post-pneumonectomy growth). Alveolar proliferation is stimulated by long-term exposure to hypoxia and is inhibited by moderate hyperoxia. The general approach of the research is to measure pulmonary growth and partitioning by using quantitative stereologic methods to study the lungs of experimental animals subjected to various treatments and conditions. In the final year of the project, for which this supplemental application is submitted, emphasis will be given to evaluating the effects of exercise, carotid body denervation and vagotomy. A comparative study of newborn lung dimensions in a variety of species will also be carried out.